A power transmitting apparatus which transmits engine power to the wheels is required not only to enable power transmission from the engine to the wheels, but to enable radial and axial displacement from the wheels as well as momentum displacement caused by bounding of the vehicle body during running on a rough road and cornering of the vehicle. Accordingly, as shown in FIG. 12, one end of the driving shaft 100 arranged between the engine and the driving wheel is connected to a differential apparatus 102 via a sliding type constant velocity universal joint 101. The other end of the shaft 100 is connected to a wheel 105 via a wheel bearing apparatus 104 which includes a stationary type constant velocity universal joint 103.
As shown in FIG. 11, this vehicle wheel bearing apparatus 104 has a wheel hub 16 on which the wheel 105 is mounted at one end. A double row rolling bearing 107 rotatably supports the wheel hub 106. An outer joint member 108, of the stationary type constant velocity universal joint 103, transmits the power of the driving shaft 100 to the wheel hub 106. The double row rolling bearing 107 has an outer member 110, a separate inner ring 109, press fit on the outer periphery of the wheel hub 106, and double row rolling elements 111 rollably contained between the wheel hub 106, the inner ring 109 and the outer member 110.
In such a vehicle wheel bearing apparatus 104, a predetermined preload is applied to the bearing apparatus in order to assure a desired bearing rigidity. Control of the bearing preload has been carried out by precisely finishing the abutting surfaces between the wheel hub 106 and the inner ring 109 as well as by tightly connecting the wheel hub 106 and the outer joint member 108 by fastening a securing nut 112 with a predetermined torque (axial force). Not only does the bearing preload influence the bearing life, it also influences other characteristics such as fuel consumption, etc. As shown in FIG. 9, since the rotational torque is proportional to the bearing preload, it is possible to contribute to improved fuel consumption by reducing the preload to reduce the rotational torque. On the contrary, since the inclination angle of bearing is an essential factor to determine the rigidity of bearing, which is inversely proportional to the bearing preload, as shown in FIG. 10, the bearing rigidity can improve by increasing the preload to reduce the inclination angle of bearing. Thus the inclination of the brake rotor (not shown) caused during cornering of the vehicle can also be suppressed.
If it is possible to control the optimum bearing preload in accordance with the running conditions of the vehicle, it is possible to provide a vehicle wheel bearing apparatus with ideal drive and fuel consumption characteristics. However no such bearing apparatus has yet been developed which can be variably set to the optimum bearing preload in accordance with the running conditions of the vehicle.
FIG. 8 shows a bearing unit 50 which can vary the bearing preload. This bearing unit has two rolling bearings 51 and 51, inner and outer cylindrical portions 52 and 53 to hold the rolling bearings 51 and 51, an electrostrictive element 54 arranged on one end of one of the cylindrical portions 52 and 53, and a voltage controller 55 to control voltage applied to the electrostrictive element 54. The cylindrical portion 52 together with the electrostrictive element 54 is secured to stationary portions 56 and 57 of the bearing unit 50. Preloading stabilization can be obtained by automatically setting the distance between the bearing inner rings at a predetermined value. A temperature sensor (not shown) detects temperature variation of the inner cylindrical portion 52 and the stationary portion 56. The distance is adjusted by extending or contracting the electrostrictive element 54 by using a voltage controller 55 to control the voltage applied to the electrostrictive element 54 in accordance with the temperature variation (cf. Japanese Laid-open Patent Publication No. 223216/1999).